Zone selective protection, also called zone selective interlock (ZSI: Zone Selectivity Interlock), is a method of reducing the impact on the various power distribution devices in a power distribution network in the event of a short circuit fault by shortening the time needed to eliminate the fault as far as possible. To be specific, suppose that a fault occurs in a multi-level power distribution network supplied by a single power source, then every circuit breaker that detects the fault must report detection of the fault to its upstream circuit breaker, i.e. send an interlock signal. Thus, only those circuit breakers that do not receive a report are determined as being the circuit breakers closest to the fault point, and implement instantaneous protection (for example, delay time t=50 ms), i.e. carry out a breaking action with the shortest delay time. Those circuit breakers which receive an interlock signal carry out timed protection, also called “locking”. By timed protection or locking is meant: if a fault has still not been eliminated after a predetermined delay time (e.g. 100 ms) has expired (e.g. the protecting action of a circuit breaker closer to the fault point has failed), then the circuit breaker in question opens; otherwise, it maintains the connection. As can be seen, ZSI enables circuit breakers to perform selective protecting actions when faults occur.
In the case of power distribution networks with multiple power sources, the simple ZSI described above is no longer suitable owing to the increased complexity of the power distribution network. This being the case, the concept of directional zone selective interlock (DISI: Directional Zone Select Interlock) is proposed, with the following basic principle: a fault point is identified on the basis of correlations among the current flow directions through the various circuit breakers when a fault occurs, and the action delay times of the various circuit breakers are then determined in order to eliminate the fault identified. The advantage of this is that selective protection can be realized more reliably even if the network contains multiple power sources or an active load.
In directional zone selective interlock, the accurate determination of the direction of current flowing through a circuit breaker when a fault occurs becomes a key problem. In the prior art, a predetermined current reference direction is generally set for each circuit breaker first. Next, to determine whether the direction of current at the present time is the same as the current reference direction, the sign (positive or negative) of the active power is found by calculation. The active power can be calculated using the following formula.
      P    m    =                    1        N            ⁢                        ∑                      k            =            0                                N            -            1                          ⁢                  (                                    v                              m                k                                      ×                          i                              m                k                                              )                      =          VI      ⁢                          ⁢      cos      ⁢                          ⁢              φ        m            wherein Pm is the active power of phase m, and m=1, 2 or 3;    N is the total number of sampling points in half a power cycle; for a power frequency of 50 Hz, half a power cycle is for example 10 ms;    k counts the sampling points in half a power cycle (0≤k≤N);    vmk is the sample voltage of phase m to the neutral line at sampling point k;    imk is the sample current of phase m at sampling point k;    V is the effective value of voltage of phase m to the neutral line over one power cycle;    I is the effective value of current of phase m over one power cycle;    φm is the phase angle between current and voltage for phase m.
The active power Pm can be calculated using the above formula. If the Pm calculated is positive, this indicates that the current flowing through the circuit breaker at the present time is in the same direction as the current reference direction, i.e. is “forward”. If the Pm calculated is negative, this indicates that the current flowing through the circuit breaker at the present time is in the opposite direction to the current reference direction, i.e. is “backward”.
When such a method of determining current direction is adopted, the time taken to determine the current direction depends to a very large degree on the frequency of the power source. For example, if the power source frequency is 50 Hz, sample values must be obtained over half a cycle before the current direction can be determined, so the time taken is theoretically at least 10 ms. In practice, taking into account the effect of short circuit current transient response, the time taken to determine the current direction is generally longer than the theoretical value of 10 ms, for example 15 ms (this is the value obtained when filter parameters are optimized to minimize the effect of short circuit current transient response).